Cytotoxic conjugates of betulinic acid and substituted triazoles prepared by Huisgen Cycloaddition from 30-azidoderivatives

In this work, we describe synthesis of conjugates of betulinic acid with substituted triazoles prepared via Huisgen 1,3-cycloaddition. All compounds contain free 28-COOH group. Allylic bromination of protected betulinic acid by NBS gave corresponding 30-bromoderivatives, their substitution with sodium azides produced 30-azidoderivatives and these azides were subjected to Cu-I catalysed Huisgen 1,3-cycloaddition to give the final conjugates. Reactions had moderate to high yields. All new compounds were tested for their in vitro cytotoxic activities on eight cancer and two non-cancer cell lines. The most active compounds were conjugates of 3 beta-O-acetylbetulinic acid and among them, conjugate with triazole substituted by benzaldehyde 9b was the best with IC50 of 3.3 mu M and therapeutic index of 9.1. Five compounds in this study had IC50 below 10 mu M and inhibited DNA and RNA synthesis and caused block in G0/G1 cell cycle phase which is highly similar to actinomycin D. It is unusual that here prepared 3 beta-O-acetates were more active than compounds with the free 3-OH group and this suggests that this set may have common mechanism of action that is different from the mechanism of action of previously known 3 beta-O-acetoxybetulinic acid derivatives. Benzaldehyde type conjugate 9b is the best candidate for further drug development.